In the field of indexable rotary cutting tools, the development of a tool having a significantly small tool diameter compared with the prior art has been desired with the aim of precisely machining a mold, etc. Such rotary cutting tool having a small diameter is generally an end mill, and the required tool diameter is as small as approximately 20 mm or less. When a conventional indexable end mill is merely reduced in terms of its diameter, the resultant diameter is extremely small as described above, which leads to a thin thickness of a back metal portion thereof, which is adjacent to a back part, with respect to a tool rotating direction, of an insert mounting seat on which a cutting insert is mounted. A back metal portion of a tool body is a portion which receives the cutting resistance applied on a cutting insert, and thus, if such portion is thin, this easily invites the generation of chattering in the cutting insert during machining. As a result, chattering easily leads to the occurrence of issues including a reduction in the quality of a surface to be machined and fracturing of a cutting insert. Conventionally, in order to solve such problems, the thickness of a back metal portion has been devised to be secured. For example, as viewed from a leading end surface side of a tool, a cutting insert is arranged such that a radial rake angle is negative while the position of a cutting corner involved in cutting is kept unchanged, whereby a back metal portion in an insert mounting seat of a tool body can be made thick.
However, when a cutting insert is arranged such that a radial rake angle is negative as described above, a bottom wall surface of an insert mounting seat faces toward an outer periphery of the tool. Thus, the cutting resistance applied on a major cutting edge which extends in a substantially axial direction on the outer periphery side of the tool causes the force received by the cutting insert to strongly include a component directed toward the outer periphery of the tool. Therefore, the cutting insert is prone to slide toward the outer periphery of the tool, which has resulted in the possibility of a reduction in the fixing property.
An example of means for solving the above problem is a method in which a lower surface of a cutting insert is formed in a convex manner so as to have a substantially V shape, as disclosed in Patent Document 1. The lower surface of the cutting insert in Patent Document 1 has a flat base surface extending in a longitudinal direction and two inclined surfaces which respectively extend outward from the sides of the base surface toward an upper surface of the cutting insert. This cutting insert is mounted on a tool body such that the base surface located at a peak portion of the V shape has a positional relationship so as to be substantially parallel to a rotational axis of the tool body. This allows an inclined surface located on the outer periphery side of the tool, from among the two inclined surfaces constituting the V shape, to receive a force directed toward the outer periphery of the tool, which suppresses shifting of the cutting insert toward the outer periphery of the tool.